Various methods have been proposed heretofore to produce or recover molybdenum oxide from starting materials including molybdenum sulfide and utilizing oxidation treatment of the molybdenum concentrate. Reference may be had to German open application 20 03 874 and German Pat. No. 28 30 394.
From these publications it is known to suspend a molybdenum sulfide concentrate, with the usual amounts of foreign matter or impurities, of a particle size range of 20 to 90 microns, in water in a slurrying or suspending stage and then to feed this aqueous suspension into an autoclave.
The autoclave constitutes the oxidation stage and, in the autoclave, the suspension is subjected at elevated temperatures to an increased oxygen partial pressure, the oxygen being, for example, bubbled through the suspension.
The molybdenum sulfide within the suspension is transformed into molybdenum oxide which remains in suspension as a solid while sulfuric acid is produced.
In a first filtering stage, the molybdenum oxide is recovered from a first filtrate containing the sulfuric acid which can be neutralized with lime or calcium carbonate to form calcium sulfate (gypsum) in a neutralization stage.
At a second filtering stage, the calcium sulfate or gypsum is filtered from the liquid phase which constitutes a second filtrate. The second filtrate can be recirculated to the suspending or slurrying stage.
In conventional apparatus for carrying out this process on stream, the output of molybdenum oxide is a function of the throughput of the apparatus and make-up water must be supplied to replace the water which is removed from the system adherent to the molybdenum oxide and the calcium sulfate.
The system of German Pat. No. 28 30 394, involves a two-stage recirculation.
In a first recirculation phase, the oxidizing suspension from the oxidation stage, prior to filtering the molybdenum oxide therefrom, is partly recirculated and combined with molybdenum sulfide concentrate for return to the autoclave. This recirculation is effected until the sulfuric acid concentration reaches 80 to 120 g/liter, whereupon molybdenum oxide is extracted from at least a portion of the output from the autoclave.
In this system, the first filtrate is treated with lime to a pH of 0.9 to 1.5 and the resulting calcium sulfate is filtered off with the second filtrate thus obtained being recirculated to the suspension or slurrying stage for combination with molybdenum sulfide concentrate and delivery to the autoclave. Obviously this results in an increase in the impurity content of the recirculated secondary filtrate with time.
It has also been suggested that the enriched second filtrate be brought to a pH of 2.5 with alkali hydroxide and this treated filtrate oxidized in an autoclave to produce iron molybdate which can be filtered therefrom with the resulting filtrate being utilized for the recovery of valuable by-products in the form of the impurities in which the second filtrate was enriched.
The aforedescribed systems had the advantage, therefore, that they permitted enrichment of the impurity levels in the secondary filtrate to concentrations which could enable the economical recovery of these impurity elements.
Experience has shown that these systems, however, are not free from disadvantages. For example, where the molybdenum oxide is recirculated, encrustation of the autoclave was to be feared and it was necessary to recirculate the filtrate in a hot state to maintain the thermal balance in the autoclave which undergoes an exothermic reaction (MoS.sub.2 +9/2 O.sub.2 +2H.sub.2 O=MoO.sub.3 +2H.sub.2 SO.sub.4).
In the earlier system, the suspension density was about 50 to 75 g of suspended solids per liter. While the prior method resulted in an effective production of molybdenum oxide, bearing in mind the limitations of throughput and the problems mentioned previously, it was found that the energy requirements of the process were excessive notwithstanding the fact that an exothermic reaction was involved.